The present invention relates to an air-fuel ratio detecting system or more in particular to a system for detecting the air-fuel ratio from the exhaust gas components produced from the engine.
Conventionally it has been proposed, as a system for detecting the air-fuel ratio from the engine exhaust gas components, e.g., an oxygen concentration, such system in which an air-fuel ratio sensor essentially comprising a metal oxide semiconductor such as titania TiO.sub.2 for exhibiting an electrical resistance depending on the oxygen concentration is connected with a fixed resistor for voltage division or a dividing resistor, and the voltage at the junction point thereof is compared with a reference voltage by a comparator circuit, and thereby it is detected whether the actual air-fuel ratio is higher (lean) or lower (rich) than the stoichiometric air-fuel ratio.
In the conventional system such as the above, however, the dividing resistor is set at a fixed value and therefore when the electrical resistance Re characteristics of the air-fuel sensor shifts from curve X (working temperature of 500.degree. C.) to curve Y (working temperature of 800.degree. C.) in FIG. 1 in accordance with the working temperature or by lapse of time, the accuracy of air-fuel detection is reduced, often leading to an erroneous detection.
The voltage V.sub.A at the junction point of the air-fuel ratio sensor and the fixed resistor, for example, changes along the curve X in FIG. 2 at the working temperature of 500.degree. C., and the crossing thereof with the reference voltage Vs shown by the solid line represents the stoichiometric air-fuel ratio ST, thus making it possible to detect whether the air-fuel ratio is higher or lower than the stoichiometric air-fuel ratio ST.
When the electrical resistance characteristic of the air-fuel sensor changes in accordance with the working temperature thereof or by lapse of time, the voltage V.sub.A changes, for instance, as shown by curve Y in FIG. 2, and the crossing with the reference voltage Vs is displaced to the lean side from the stoichiometric air-fuel ratio ST by .DELTA.A/F, thus it causes an error in the detection of the air-fuel ratio. In an extreme case, the crossing between the voltage V.sub.A and reference voltage Vs disappears, thus making it impossible to detect the air-fuel ratio.